Monsanto hit out after the Assembly Government slammed the door shut on any lingering hopes of introducing GM crops in Wales.

New regulations, if adopted, will set Wales apart from England in applying a strict “polluter pays” principle that will ban even trial plantings.

But a Monsanto spokesman said the proposal was inconsistent with EU guidelines on co-existence with conventional and organic crops.

It would end up destroying the Welsh livestock industry, he said.

“Approximately 85% of compound animal feed throughout the UK already contains imported GM ingredients, due to the large shortage of home-produced protein,” he said.

“We would have concern that in the long run this proposal would put the majority of Welsh livestock farmers at a serious competitive disadvantage, and merely drive livestock production overseas.”

GM companies have long resisted efforts to make them responsible for “leaks” of GM material and concurrent Defra proposals for England stop short of pinning liability on the operator in the event of environmental or economic damage.

But Cardiff’s proposals would make GM companies like Monsanto and Bayer, and GM crop growers, legally liable for contamination or “genetic trespass”. Such liability would apply even if they have a licence and even if scientific knowledge at the time leads them to believe the material was harmless.

The move puts clear water between the administrations in Cardiff Bay and London and takes Welsh opposition to GM science to new levels.

English GM crop farmers would become liable if they contaminated land in Wales but Ms Jones said she did not foresee any difficulties in implementation.

“Border issues will arise along any boundaries between EU countries, but it’s the right of this Assembly Government to exercise the powers we have to pursue our political aspiration,” she said.

Supporters of the proposal include the Farmers’ Union of Wales, which is a member of the GM Free Alliance - a group of organisations which includes the RSPB, Friends of the Earth Cymru and GM Free Cymru.

FUW vice-president Brian Walters said the union was concerned by the threat of cross-contamination.

“It would be completely unfair if non-GM farmers’ incomes suffered as a result of wind or insect-borne cross-pollination that was beyond their control,” said Mr Walters. “The draft Welsh regulations provide security for Welsh farmers, whereas Defra has left English farmers out in the cold.”

The GM Free Wales Alliance described Cardiff’s proposal as “the latest step in the protection of Welsh farming and the Welsh environment”.

Brian John, of GM Free Cymru, said he expected the devolved administrations in Scotland and Northern Ireland to follow suit.

“There is a degree of frustration that Westminster continues to vote in favour of GM approval at EU level despite the reservations that the other three authorities have,” said Mr John.

“Westminster always pushes a pro-GM agenda in the EU, despite the fact that it’s not the majority view but a distinctly English line.”

The Monsanto spokesman said the cultivation of GM crops is increasing world-wide, with more than 100m hectares grown every year by 10m farmers.

“Most of the world is moving on from the tired old debate of 10 years ago and accepting that biotechnology has a place to play alongside a range of farming methods,” he added.